Flink on YARN模式下TaskManager的内存分配探究

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我们使用如下的参数提交了Flink on YARN作业（per-job模式）。

/opt/flink-1.9.0/bin/flink run \
--detached \
--jobmanager yarn-cluster \
--yarnname "x.y.z" \
--yarnjobManagerMemory 2048 \
--yarntaskManagerMemory 4096 \
--yarnslots 2 \
--parallelism 20 \
--class x.y.z \
xyz-1.0.jar

该作业启动了10个TaskManager，并正常运行。来到该任务的Web界面，随便打开一个TaskManager页面，看看它的内存情况。

可见，虽然我们在参数中设置了TaskManager的内存为4GB大，但是上图显示的JVM堆大小只有2.47GB，另外还有一项“Flink Managed Memory”为1.78GB。在用VisualVM监控YarnTaskExecutorRunner时，会发现其JVM内存参数被如下设置：

显然Xmx+MaxDirectMemorySize才是我们在启动参数中设定的TM内存大小（4GB）。那么为什么会这样设置？“Flink Managed Memory”又是什么鬼？下面就来弄懂这些问题。

TaskManager内存布局如下图所示。

为了减少object overhead，Flink主要采用序列化的方式存储各种对象。序列化存储的最小单位叫做MemorySegment，底层为字节数组，大小由taskmanager.memory.segment-size参数指定，默认32KB大。下面分别介绍各块内存：

• 网络缓存（Network Buffer）：用于网络传输及与网络相关的动作（shuffle、广播等）的内存块，由MemorySegment组成。从Flink 1.5版本之后，网络缓存固定分配在堆外，这样可以充分利用零拷贝等技术。与它相关的三个参数及我们的设定值如下：

# 网络缓存占TM内存的默认比例，默认0.1
taskmanager.network.memory.fraction: 0.15
# 网络缓存的最小值和最大值 ，默认64MB和1GB
taskmanager.network.memory.min: 128mb
taskmanager.network.memory.max: 1gb

• 托管内存（Flink Managed Memory）：用于所有Flink内部算子逻辑的内存分配，以及中间数据的存储，同样由MemorySegment组成，并通过Flink的MemoryManager组件管理。它默认在堆内分配，如果开启堆外内存分配的开关，也可以在堆内、堆外同时分配。与它相关的两个参数如下：

# 堆内托管内存占TM堆内内存的比例，默认0.7
taskmanager.memory.fraction: 0.7
# 是否允许分配堆外托管内存，默认不允许
taskmanager.memory.off-heap: false

由此也可见，Flink的内存管理不像Spark一样区分Storage和Execution内存，而是直接合二为一，更加灵活。

• 空闲内存（Free）：虽然名为空闲，但实际上是存储用户代码和数据结构的，固定在堆内，可以理解为堆内内存除去托管内存后剩下的那部分。

如果我们想知道文章开头的问题中各块内存的大小是怎么来的，最好的办法自然是去读源码。下面以Flink 1.9.0源码为例来探索。

TaskManager内存分配逻辑

YARN per-job集群的启动入口位于o.a.f.yarn.YarnClusterDescriptor类中。

    public ClusterClient<ApplicationId> deployJobCluster(ClusterSpecification clusterSpecification,JobGraph jobGraph,boolean detached) throws ClusterDeploymentException {// this is required because the slots are allocated lazilyjobGraph.setAllowQueuedScheduling(true);try {return deployInternal(clusterSpecification,"Flink per-job cluster",getYarnJobClusterEntrypoint(),jobGraph,detached);} catch (Exception e) {throw new ClusterDeploymentException("Could not deploy Yarn job cluster.", e);}}

其中，ClusterSpecification对象持有该集群的4个基本参数：JobManager内存大小、TaskManager内存大小、TaskManager数量、每个TaskManager的slot数。而deployInternal()方法在开头调用了o.a.f.yarn.AbstractYarnClusterDescriptor抽象类的validateClusterSpecification()方法，用于校验ClusterSpecification是否合法。

    private void validateClusterSpecification(ClusterSpecification clusterSpecification) throws FlinkException {try {final long taskManagerMemorySize = clusterSpecification.getTaskManagerMemoryMB();// We do the validation by calling the calculation methods here// Internally these methods will check whether the cluster can be started with the provided// ClusterSpecification and the configured memory requirementsfinal long cutoff = ContaineredTaskManagerParameters.calculateCutoffMB(flinkConfiguration, taskManagerMemorySize);TaskManagerServices.calculateHeapSizeMB(taskManagerMemorySize - cutoff, flinkConfiguration);} catch (IllegalArgumentException iae) {throw new FlinkException("Cannot fulfill the minimum memory requirements with the provided " +"cluster specification. Please increase the memory of the cluster.", iae);}}

ClusterSpecification.getTaskManagerMemoryMB()方法返回的就是-ytm/--yarntaskManagerMemory参数设定的内存，最终反映在Flink代码中都是taskmanager.heap.size配置项的值。

接下来首先调用ContaineredTaskManagerParameters.calculateCutoffMB()方法，它负责计算一个承载TM的YARN Container需要预留多少内存给TM之外的逻辑来使用。

    public static long calculateCutoffMB(Configuration config, long containerMemoryMB) {Preconditions.checkArgument(containerMemoryMB > 0);// (1) check cutoff ratiofinal float memoryCutoffRatio = config.getFloat(ResourceManagerOptions.CONTAINERIZED_HEAP_CUTOFF_RATIO);if (memoryCutoffRatio >= 1 || memoryCutoffRatio <= 0) {throw new IllegalArgumentException("The configuration value '"+ ResourceManagerOptions.CONTAINERIZED_HEAP_CUTOFF_RATIO.key() + "' must be between 0 and 1. Value given="+ memoryCutoffRatio);}// (2) check min cutoff valuefinal int minCutoff = config.getInteger(ResourceManagerOptions.CONTAINERIZED_HEAP_CUTOFF_MIN);if (minCutoff >= containerMemoryMB) {throw new IllegalArgumentException("The configuration value '"+ ResourceManagerOptions.CONTAINERIZED_HEAP_CUTOFF_MIN.key() + "'='" + minCutoff+ "' is larger than the total container memory " + containerMemoryMB);}// (3) check between heap and off-heaplong cutoff = (long) (containerMemoryMB * memoryCutoffRatio);if (cutoff < minCutoff) {cutoff = minCutoff;}return cutoff;}

该方法的执行流程如下：

1. 获取containerized.heap-cutoff-ratio参数，它代表Container预留的非TM内存占设定的TM内存的比例，默认值0.25；

2. 获取containerized.heap-cutoff-min参数，它代表Container预留的非TM内存的最小值，默认值600MB；

3. 按比例计算预留内存，并保证结果不小于最小值。

由此可见，在Flink on YARN时，我们设定的TM内存实际上是Container的内存。也就是说，一个TM能利用的总内存（包含堆内和堆外）是：

tm_total_memory = taskmanager.heap.size - max[containerized.heap-cutoff-min, taskmanager.heap.size * containerized.heap-cutoff-ratio]

用文章开头给的参数实际计算一下：

tm_total_memory = 4096 - max[600, 4096 * 0.25] = 3072

接下来看TaskManagerServices.calculateHeapSizeMB()方法。

    public static long calculateHeapSizeMB(long totalJavaMemorySizeMB, Configuration config) {Preconditions.checkArgument(totalJavaMemorySizeMB > 0);// all values below here are in bytesfinal long totalProcessMemory = megabytesToBytes(totalJavaMemorySizeMB);final long networkReservedMemory = getReservedNetworkMemory(config, totalProcessMemory);final long heapAndManagedMemory = totalProcessMemory - networkReservedMemory;if (config.getBoolean(TaskManagerOptions.MEMORY_OFF_HEAP)) {final long managedMemorySize = getManagedMemoryFromHeapAndManaged(config, heapAndManagedMemory);ConfigurationParserUtils.checkConfigParameter(managedMemorySize < heapAndManagedMemory, managedMemorySize,TaskManagerOptions.MANAGED_MEMORY_SIZE.key(),"Managed memory size too large for " + (networkReservedMemory >> 20) +" MB network buffer memory and a total of " + totalJavaMemorySizeMB +" MB JVM memory");return bytesToMegabytes(heapAndManagedMemory - managedMemorySize);}else {return bytesToMegabytes(heapAndManagedMemory);}}

为了简化问题及符合我们的实际应用，就不考虑开启堆外托管内存的情况了。这里涉及到了计算Network buffer大小的方法```

NettyShuffleEnvironmentConfiguration.calculateNetworkBufferMemory()。public static long calculateNetworkBufferMemory(long totalJavaMemorySize, Configuration config) {final int segmentSize = ConfigurationParserUtils.getPageSize(config);final long networkBufBytes;if (hasNewNetworkConfig(config)) {float networkBufFraction = config.getFloat(NettyShuffleEnvironmentOptions.NETWORK_BUFFERS_MEMORY_FRACTION);long networkBufSize = (long) (totalJavaMemorySize * networkBufFraction);networkBufBytes = calculateNewNetworkBufferMemory(config, networkBufSize, totalJavaMemorySize);} else {// use old (deprecated) network buffers parameter// 旧版逻辑，不再看了}return networkBufBytes;}private static long calculateNewNetworkBufferMemory(Configuration config, long networkBufSize, long maxJvmHeapMemory) {float networkBufFraction = config.getFloat(NettyShuffleEnvironmentOptions.NETWORK_BUFFERS_MEMORY_FRACTION);long networkBufMin = MemorySize.parse(config.getString(NettyShuffleEnvironmentOptions.NETWORK_BUFFERS_MEMORY_MIN)).getBytes();long networkBufMax = MemorySize.parse(config.getString(NettyShuffleEnvironmentOptions.NETWORK_BUFFERS_MEMORY_MAX)).getBytes();int pageSize = ConfigurationParserUtils.getPageSize(config);checkNewNetworkConfig(pageSize, networkBufFraction, networkBufMin, networkBufMax);long networkBufBytes = Math.min(networkBufMax, Math.max(networkBufMin, networkBufSize));ConfigurationParserUtils.checkConfigParameter(/*...*/);return networkBufBytes;}

由此可见，网络缓存的大小这样确定：

network_buffer_memory = min[taskmanager.network.memory.max, max(askmanager.network.memory.min, tm_total_memory * taskmanager.network.memory.fraction)]

代入数值：

network_buffer_memory = min[1024, max(128, 3072 * 0.15)] = 460.8

也就是说，TM真正使用的堆内内存为：

tm_heap_memory = tm_total_memory - network_buffer_memory = 3072 - 460.8 ≈ 2611

这完全符合VisualVM截图中的-Xms/-Xmx设定。

同理，可以看一下TaskManager UI中的网络缓存MemorySegment计数。

通过计算得知，网络缓存的实际值与上面算出来的network_buffer_memory值是非常接近的。

那么堆内托管内存的值是怎么计算出来的呢？前面提到了托管内存由MemoryManager管理，来看看TaskManagerServices.createMemoryManager()方法，它用设定好的参数来初始化一个MemoryManager。

private static MemoryManager createMemoryManager(TaskManagerServicesConfiguration taskManagerServicesConfiguration) throws Exception {long configuredMemory = taskManagerServicesConfiguration.getConfiguredMemory();MemoryType memType = taskManagerServicesConfiguration.getMemoryType();final long memorySize;boolean preAllocateMemory = taskManagerServicesConfiguration.isPreAllocateMemory();if (configuredMemory > 0) {if (preAllocateMemory) {LOG.info(/*...*/);} else {LOG.info(/*...*/);}memorySize = configuredMemory << 20; // megabytes to bytes} else {// similar to #calculateNetworkBufferMemory(TaskManagerServicesConfiguration tmConfig)float memoryFraction = taskManagerServicesConfiguration.getMemoryFraction();if (memType == MemoryType.HEAP) {long freeHeapMemoryWithDefrag = taskManagerServicesConfiguration.getFreeHeapMemoryWithDefrag();// network buffers allocated off-heap -> use memoryFraction of the available heap:long relativeMemSize = (long) (freeHeapMemoryWithDefrag * memoryFraction);if (preAllocateMemory) {LOG.info(/*...*/);} else {LOG.info(/*...*/);}memorySize = relativeMemSize;} else if (memType == MemoryType.OFF_HEAP) {long maxJvmHeapMemory = taskManagerServicesConfiguration.getMaxJvmHeapMemory();// The maximum heap memory has been adjusted according to the fraction (see// calculateHeapSizeMB(long totalJavaMemorySizeMB, Configuration config)), i.e.// maxJvmHeap = jvmTotalNoNet - jvmTotalNoNet * memoryFraction = jvmTotalNoNet * (1 - memoryFraction)// directMemorySize = jvmTotalNoNet * memoryFractionlong directMemorySize = (long) (maxJvmHeapMemory / (1.0 - memoryFraction) * memoryFraction);if (preAllocateMemory) {LOG.info(/*...*/);} else {LOG.info(/*...*/);}memorySize = directMemorySize;} else {throw new RuntimeException("No supported memory type detected.");}}// now start the memory managerfinal MemoryManager memoryManager;try {memoryManager = new MemoryManager(memorySize,taskManagerServicesConfiguration.getNumberOfSlots(),taskManagerServicesConfiguration.getPageSize(),memType,preAllocateMemory);} catch (OutOfMemoryError e) {// ...}return memoryManager;}

简要叙述一下流程：

1. 获取taskmanager.memory.size参数，用来确定托管内存的绝对大小；

2. 如果taskmanager.memory.size未设置，就继续获取前面提到过的taskmanager.memory.fraction参数；

3. 只考虑堆内内存的情况，调用TaskManagerServicesConfiguration.getFreeHeapMemoryWithDefrag()方法，先主动触发GC，然后获取可用的堆内存量。可见，如果没有意外，程序初始化时该方法返回的值与前文的-Xms/-Xmx应该相同；

4. 计算托管内存大小和其他参数，返回MemoryManager实例。

一般来讲我们都不会简单粗暴地设置taskmanager.memory.size。所以：

flink_managed_memory = tm_heap_memory * taskmanager.memory.fraction = 2611 * 0.7 ≈ 1827

这就是TaskManager UI中显示的托管内存大小了。

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